From the very
beginning, radio communications used Morse code for data communications.
Over time, improved techniques were developed for data transmission that
take into account the variability of the radio medium and greatly increase
the speed at which data transmission occurs over a radio link. In addition,
the application of error- correcting codes and automatic repeat request
(ARQ) techniques offering error- free data transfer permits the use of
radio transmissions for computer- to- computer communications systems.

To understand
the principles of radio data communication, we’ll definesome common
data terminology and explain the significance of the modem. We will also
outline some of the problems and solutions associated with radio data communication.

Binary Data

Communication
as an activity involves the transfer of information froma transmitter
to a receiver over a suitable channel. Consider this book, for instance.
It uses symbols (the alphabet) to encode information into a set of code
groups (words) for transmission over a channel (the printed page) to a
receiver (the reader). Applying this principle to data (information), we
begin by using a kind of shorthand to transform the data into code words
(binary digits or bits) for transmission over a channel (HF radio) to a
receiver (the reader).

Bits are part
of a number system having a base of two that uses only thesymbols 0
and 1. Thus, a bit is any variable that assumes two distinct states. For
example, a switch is open or closed; a voltage is positive ornegative,
and so on.

A simple way
to communicate binary data is to switch a circuit off and onin patterns
that are interpreted at the other end of a link. This is essentially what
was done in the early days of telegraphy. Later schemes used a bit to select
one of two possible states of the properties that characterize a carrier
(modulated radio wave) — either frequency or amplitude. More sophisticated
approaches allow the carrier to assume more than two states and hence to
represent multiple bits.

Baud Rate

Data transmission
speed is commonly measured in bits per second (bps).Sometimes
the word baud is used synonymously with bps, although the two terms actually
have different meanings. Baud is a unit of signaling speed and is a measure
of symbols per second that are being sent. A symbol may represent more
than one bit.

The maximum
baud rate supported by a radio channel depends on itsbandwidth
— the greater the bandwidth, the greater the baud rate. Therate at which
information is transmitted, the bit rate, depends on howmany bits
there are per symbol.

Asynchronous
and Synchronous Data

The transmission
of data occurs in either an asynchronous or synchronousmode. In
asynchronous data transmission, each character has a start and stop bit
(Figure 5- 1). The start bit prepares the data receiver to accept the character.
The stop bit brings the data receiver back to an idle state.

Synchronous
data transmission eliminates the start and stop bits. This typeof system
uses a preamble (a known sequence of bits, sent at the start of amessage,
that the receiver uses to synchronize to its internal clock) to alert the
data receiver that a message is coming. Asynchronous systems eliminate
the need for complex synchronization circuits, but at the cost of higher
overhead than synchronous systems. The stop and start bits increase the
length of a character by 25 percent, from 8 to 10 bits.

Radio Modems

Radios cannot
transmit data directly. Data digital voltage levels must beconverted
to radio signals, using a device called a modulator, which applies the
audio to the transmitter. Conversely, at the receiver, a demodulator converts
audio back to digital voltage levels. The Harris radios are equipped with
built- in high- speed modems (the MOdulator and the DEModulator packaged
together), which permit the radios to operate with either voice or data
inputs.

The simplest
modems employ FSK to encode binary data (0s and 1s) (seeFigure 5-
2). The input to the modulator is a digital signal that takes oneof two possible
voltage levels. The output of the modulator is an RF signalthat is one
of two possible tones. FSK systems are limited to data ratesless than
75 bps due to the effects of multipath propagation.

Amplitude
Shift Keying (ASK) is similar to FSK except that it is the amplitude of
the carrier that is modulated rather than the frequency.Higher rates
are possible with more modern Phase Shift Keying (PSK)modulation
methods and advanced coding schemes. PSK is describedlater in
this chapter.

Error Control

There are
several different approaches to avoid data transmission problems.

Forward Error
Correction (FEC) adds redundant data to the data streamto allow
the data receiver to detect and correct errors. An importantaspect of
this concept is that it does not require a return channel forthe acknowledgment.
If a data receiver detects an error, it simply correctsit and accurately
reproduces the original data without notifying the datasender that
there was a problem. Downsides of FEC: Unlike ARQ, FEC does not ensure
error- free data transmission; FEC decreases the effective data throughput.

The FEC coding
technique is most effective if errors occur randomly in adata stream.
The radio medium, however, typically introduces errors that occur in bursts
— that is, intervals with a high bit error ratio (BER) in the channel are
interspersed with intervals of a low BER. To take full advantage of the
FEC coding technique, it’s best to randomize the errors that occur in the
channel by a process called interleaving (Figure 5- 3).

For example,
at the modulator, the data stream enters a 9- row by 10- column matrix.
The blocks are entered by rows and unloaded by columns. When the data stream
leaves the matrix for transmission, the sequence of output bits will be
1, 11, 21, and so on.

At the demodulator,
de- interleaving reverses the process. Data is enteredby columns
in a matrix identical to that at the transmitter. It is read out in rows,
restoring the sequence of data to its original state. Thus, if a burst
were to cause 9 consecutive bits to be in error, no more than 3 of them
will fall in any 30- bit sequence of bits after de- interleaving.

Then, if an
FEC coding technique were used, the errors would be corrected.Soft- decision
decoding further enhances the power of the error- correctioncoding. In
this process, a group of detected symbols that retain theiranalog character
are compared against the set of possible transmittedcode words.
The system “remembers” the voltage from the detector and applies a weighing
factor to each symbol in the code word before making a decision about which
code word was transmitted.

Vocoder

Data communications
techniques are also used for encrypting voice callsby a device
called a vocoder (short for voice coder- decoder). The vocoderconverts
sound into a data stream for transmission over an HF radio channel. A vocoder
at the receiving end reconstructs the data intotelephone-
quality sound.

Channel Equalization
and Excision Filtering

In addition
to error correction techniques, high- speed serial modems mayinclude two
signal- processing schemes that improve data transmissions.An automatic
channel equalizer compensates for variations in the channelcharacteristics
as data is being received. An adaptive excision filter seeks out and suppresses
narrowband interference in the demodulator input,reducing
the effects of co- channel interference, that is, interference on the same
channel that is being used. Harris has patented several techniquesto perform
these functions.

Modern High
Data Rate Modem Waveforms

High- speed
modem technology, permits data rates as high as 64 kbps. Radio transmission
paths have varying characteristics depending upon the frequency band (HF,
VHF, and UHF) and the bandwidth of the channel.Although
most HF channels are bandwidth limited to 3 kHz; VHF, UHF, andSATCOM channels
have both 5 kHz and 25 kHz bandwidths. To accommodate and maximize the
data throughput rate for these radio transmission types, a number of robust
data waveforms have been created. Table 5-1 lists these different waveform
types and their applications.

Phase Shift
Keying (PSK)

PSK is similar
to FSK, shown in Figure 5- 2, except that it is the phase of the carrier
rather than the frequency that is modulated.

Binary Phase
Shift Keying (BPSK)

The simplest
form of PSK is called Binary Phase Shift Keying (BPSK) shownin Figure
5- 4. Figure 5- 4a shows a reference wave covering two bit periods. Figure
5- 4b shows the wave after modulation with a (0) bit and a (1) bit. Notice
that the signal corresponding to the second bit (1) is an upside-down version
of the reference waveform. This portion of the signal is 180° with
respect to the reference waveform.

Notice also
that the transition from the first bit to the second is abrupt. This sudden
phase discontinuity creates a burst of noise sidebands referred to as “splatter.”
This noise causes inter- symbol interference which severely limits the
data rate that this simple form of PSK can deliver.M- ary PSK
There are many forms of PSK. BPSK is modulated with just two phases of
the carrier. Another term for BPSK is 2- ary PSK. In this case M= 2.

Figure 5-
5 shows a diagram that represents M- ary PSK by showing vectors that represent
the phase angles associated with the most common types ofM- ary PSK
modulation. BPSK is represented by two arrows facing away from each other
at a 180° angle. Each of the two phases of BPSK can represent only
one bit of information, either a (0) or a (1).

Quadrature
Phase Shift Keying (QPSK), or 4- ary PSK, is shown with fourarrows arranged
around a circle so that each is 45° apart. Since there arefour phase
states used in this modulation, each of these phases canrepresent
two bits of information. Going clockwise around the circle,these bits
are (00), (01), (10), and (11). This multi- bit representation per phase
is the key to faster data rates, because each phase representstwo bits
rather than just one. The figure also shows 8- ary PSK modulation, in which
each phase represents three bits. Finally, 16- ary PSK is shown. Each phase
represents four bits of information. On a non- noisy radio channel, 16-
ary PSK has a data rate that is four times faster than BPSK because each
modulation phase state represents four times as many bits.

Continuous
Phase QPSK

Figure 5-
6a shows what the waveforms of QPSK look like for each of thefour possible
modulation states of (00), (10), (10), and (11). Each of these bit pairs
represents a code symbol.

Figure 5-
6b shows a QPSK waveform covering two symbol periods in whichthe symbols
change from (00) to (10). Notice that although this requiresan 180°
shift, there is no sudden discontinuity in the waveform. This isbecause a
transition period equal to half of the symbol period has beentaken to
gradually change the phase. Although this slows down the datarate, the
extra time is made up by the decrease in discontinuity noise(splatter)
and attendant inter symbol interference.

Noise Margin

The problem
with PSK waveforms with M = to 8 or 16 is that the differencein phase
between each modulation state is very small. For example, in 8- ary and
16- ary PSK, the phase difference between the (0000) and (0001)symbols is
only 45° and 22.5°, respectively. The noise margin is only halfof those
values because any noise that would make the signal appear to behalf way
between the true values would yield a doubtful decision. Thus thenoise margin
for 8- ary and 16- ary PSK is only 22.5° and 12.5°, respectively.

In a noisy
radio channel, such a narrow phase difference is much harder todetect than
the 90° noise margin of the two possible phase states in BPSK for the
symbols (0) and (1). So, although 16- ary PSK can be four times as fast
as BPSK in a perfect channel, it may be totally unreadable in a noisy channel.

The phase
difference between adjacent phase states in a PSK scheme iscalled its
“noise margin”. The greater this noise margin, the more immuneto noise
this symbol transition is.

BPSK may be
slow, but it is very robust in a noisy channel.

Trellis Coded
Modulation (TCM)

Figure 5-
7 (A0) is a representation of an 8- ary PSK phase diagram where the linear
distance between the arrows of adjacent phase points is labeled (d). As
mentioned above, the noise margin corresponding to this distance is22.5°.
The term “distance” is another way of referring to noise margin.

The distance
between successive symbols in a data stream can be maximized by partitioning
into code subsets having increasing distance between their elements. Starting
from 8- PSK constellation (in Figure 5- 7 A0), we can create two 4- PSK
subsets by taking every other signal point on the circle and putting them
in one set and the rest of the signal points into another set (sets B0
and B1). The distance between adjacent phases on each of these sets is
1.85 times (d).

Each of the
resulting 4- PSK sets can be further partitioned into twoBPSK subsets
(C0, C1, and C2, C3). The distance between the two signalpoints in
each BPSK subset is 2.6 times (d). Considering all combinationsof phases
for each constellation, there are a total of six subsets of thebasic 8-
PSK signal set.

Each choice
of subset, including the choice of one of the BPSK symbolsin the last
set, is assigned a bit value for a total of three bits. Becauseeach bit
has a different signal distance associated with it, each bit hasa different
likelihood of error.

The bits with
the highest likelihood of error are coded into subsets with agreater distance
between bits. The effect of coding is to make the signaldifferent
over multiple symbols due to the bit input at the present symbol.Distance
is now measured over the several symbol intervals allowing thesignal to
“build up” more distance for any bit decision.

This process
of subset partitioning and coding is called Trellis CodedModulation.
This basic concept can be extended to a 16- ary PSK signalwith a bit
rate of up to 64 kbps in a 25 kHz bandwidth radio channel.

SUMMARY

The transmission
of data requires the use of modems to convert digitaldata RF signal
form when transmitting, and convert the RF signal backto digital
form when receiving.